Electron discharge devices



Oct. 31, 1961 A. H. w. BECK Re. 25,070

' ELECTRON DISCHARGE DEVICES Original Filed Aug. 1, 1950 3 Sheets-Sheet1 Inventor ARNOLD H- W. BECK Attorney Oct. 31, 1961 A. H. w. BECKELECTRON DISCHARGE DEVICES 3 Sheets-Sheet 2 Original Filed Aug. 1, 1950Inventor ARNOLD H IMBECK Attorney Oct. 31, 1961 A. H. w. BECK ELECTRONDISCHARGE DEVICES 3 Sheets-Sheet 3 Inventor ARNOLD H. W. BECK B, 41,

Attorney ,mo Jam a States Patent 25,070 ELECTRON DISCHARGE DEVICESArnold 11 W- Beds Tl'llmpingtou, Cambridge, England,

was to. International Standard Electric Corpora- New Yor N.,Y., acorporation of Delaware Griglnal No. 2,829,299, dated Apr. 1, 195a, Ser.No.

176,925, Aug. 1', 1950. A lication for reissue Aug.

28, 1959, Ser. No. 836,835? um piiorlty, agitation Great Britain Aug.12, 1949 7 aim. or. sis-3.5

Matter. enclosed in heavy brackets II II appears in the atent but formsno partof this reissue s ecifirnatter printed in italics indicates theadditions ma e by reissue- The present invention relates to electronbeam arrangements where the beam current is limited by space charge andis particularly concerned with the focusing of electron through devicessuch as electron velocity modulatiou tubes where the maximum possiblebeam current is ifi quired. g

p In most devices utilising an electron beam, the design problem isthatof forcing a beam througha tunnel Withdut loss of electrons to the wallsthereof. Her'etofore, beam focusing arrangements have been either of th:magnetic or of the electrostatic kind. In the case of electro'staticfocusing, if we assume that we are able to set up electric fields whichcause electrons emitted from a source, such as a thermionic cathode, toconverge so afh to graze the entrance aperture of the tunnel, the mutualrepulsion between the electrons in the field free within thetnnnel willcause their eventual diverseries so that a throat is formed at which theelectron beam has a minimum cross-section and the electrons aretravelling substantially parallel to the axis of the tunnel. The of thebeam is symmetrical to either side of the" maximum current that can beforced the tunnel without collection by the walls is that the electronbeam just grazes the entrance and as b -flutes. v v

'llms; for any given input aperture and initial axial velocity there isa maximum current that can be projected'throngir the tunnel without lossto the walls. This is'ti'he whatever the shape of the cross-section ofalso applies to systems having radial symab'ont a' c'enti'al cathode soas to produce a fanped, radiating electron beam. For a circular tunnel(If Cl and length lit is known that is seam current measured in amperesand 45- isthe'betrm potential measured in volts. For a rettmg-nsr' ofcross-section ax b' the present inventot shown that the maximum currentis given by It the specification we shall be concerned with aredlecu'oit eiectro'statically focused to form a throat its describedabove, and the expression elecfibstifie' throat.- is taken to indicatethat transverse crossslzt'nw of in eieecrost'aticai ly focused beam in afield flnetunnul at which the radial velocity components of bellman:reduced substantially to zero, the beam living ilit grazed the inputaperture of the tunnel. In the case bf't cylindrical tunnel it is knownthat the diameter the'idfint aperture is 2.4 times the diameter oldiethreat. a

' If magnetic focusing beemployed, the individual elecumsuav'eiinhelicul paths (assuming that at the entrance t8 the tunnel the beatnisstrictly parallel to the tunnel ajris) and in theory, given asufficiently strong magnetic fliid'; seam cross section may be made toremain constant along any length of tunnel. In practice, there will besome electrons having out-ward radial velocity components due to suchrandom causes as thermal agitation, with the result that a longitudinalcross-section of the beam is bounded by a pair of semi-cycloids and themagnetic field can be adjusted so that the beam has minimum transversecross-sections at the entrance and exit apertures. The beam potential,however, or what amounts to the same thing, the axial velocity of theindividual electrons, is not constant across any transversecross-section of the beam but falls to a minimum at the center of thecross-section. If the potential falls to zero, i.e. the electrons at thecenter have zero axial velocity, we are left with a stationary spacecharge or virtual cathode. Any further attempt to force more electronsof the same initial axial velocities through the tunnel results in adenser change formation without increase of the current issuing from theexit aperture of the tunnel. For most practical purposes, when the beamis to be used in an electron velocity modulation device, the potentialfall on the axis must be limited to some 10%, and for this value ofpotential variation in a cylindrical tunnel using magnetic focusing, themaximum current that can be projected through the tunnel is given by6.26X l0 V (3) It is to be noted that, provided the length of the tunnelis greater than its diameter, this maximum current is independent of thetunnel length; on the other hand the magnetic field required to preventthe beam from spreading is a function of the current.

In comparing electrostatic and magnetic focusing, we see that while inthe former case, for a given tunnel diameter and beam voltage, thelength of the tunnel is limited by the current which is to be passedthrough it, a convergent beam is required at the tunnel entrance. Thismeans that we can employ a cathode of larger area than the tunnelcross-section with consequent reduction of cathode loading. Withmagnetic focusing, on the other hand, we are not restricted as to lengthof tunnel; but the beam must be parallel to the axis of the tunnel atits entrance which implies that the cathode area must not be greaterthan that of the cross-section of the tunnel entrance. Hence, withmagnetic focusing, for the same voltage and tunnel diameter we must havea higher cathode loading than in the electrostatic case.

The present invention combines electrostatic and magnetic focusing so asto enable a reduction in cathode loading by using a convergent beam forentry into the system, while allowing a tunnel to be used whose lengthis not dependent upon the current to be forced through it. Accordingly,the present invention provides an electron beam focusing arrangementadapted to focus a spacechargc-lirnited current through a tunnel inwhich divergence of the beam is counteracted by means of an axialmagnetic field, characterised in this, that the beam is eiectrostatically' focused to' converge from an electron source and to be substantiallyparallel to the axis of the tunnel at the entrance aperture thereof, thebeam being substantially free from the influence of the said magneticfield prior to crossing the said entrance aperture. This last conditionis essential if we wish to employ purely electrostatic means to guidethe beam into the tunnel. The most direct way of ensuring thiselectromagnetic screening is to place the electrostatic focusing P rtionof the system within one of the pole pieces between which the magneticfield is set up.

The invention will be described with reference to the accompanyingdrawings in which;

- FIG. 1 illustrates diagrammatically, for purposes of explanation, theessential parts of a known electrostatic focusing system;

FIG. 2 represents diagrammatically an enlarged crosssectional view of anelectron beam focusing system according to the present invention, and

FIG. 3 shows a longitudinal cross-section in part diagrammatic, of anembodiment of the invention in an electron velocity modulation device,and

FIG. 4 shows a longitudinal cross-section in part diagrammatic, ofanother embodiment.

In FIG. 1 reference numeral 1 indicates a tunnel which may form, forexample, a drift tube in an electron velocity modulation device betweengaps 2 and 3 in which interaction between the beam and electromagneticfields takes place. The beam is converged into the entrance of thetunnel by the electron gun 4, which is shown as comprising a cathode 5and focusing cylinder 6. A collector electrode is indicated at 7. Theprofile of the electron beam is shown at 8 and narrows down to anelectrostatic throat having a diameter d. It will be seen that theprofile of the beam just grazes the entrance and exit apertnres at thegaps 2 and 3. From Formula I quoted above, since we have here designatedd as the diameter of the throat rather than that of the tunnel, theexpression for the maximum current through the tunnel In FIG. 2 thetunnel 9 of length L is shown positioned between two pole pieces 10 and11, marked conventionally N and S respectively, leaving small gaps 12and 13 for interaction purposes. Pole piece 10 is shown hollow andcontains an electron gun 14 comprising cathode 15 and focusing electrode16. The inner surface of pole piece 10 may conveniently be made to formthe anode of the electron gun, and also to form an electrostatic tunnelof entrance diameter d so that the electron beam whose profile isindicated at 17 may form an electrostatic throat of diameter d at thegap 12. Since the space inside the electrostatic tunnel is substantiallyfield free, the wall may conveniently be narrowed down from the diameterd' to d without in any way altering the effect upon the beam. Theelectron gun 14 may be designed according to well known principles toprovide the necessary converging current at the entrance of theelectrostatic tunnel. The construction illustrated is probably thesimplest method of obtaining a parallel beam at the gap 12, althoughspecial guns having shaded pole pieces could be designed for the samepurpose. The thickness of the front wall of the pole piece 10 is l/ 2,so that the pole piece forms one half of an electrostatic tunnel such asshown in FIG. 1. The pole piece 11 may conveniently contain a recess 18in which the electrons are finally collected. The diameter D of tunnel 9is shown larger than that of the entrance aperture diameter d in orderto allow for the divergence of electrons which have small radialvelocities at the entrance aperture, as discussed above in connectionwith magnetic focusing. In an electron velocity modulation device thelength L and the width of the beam at the gaps 12 and 13 are determinedby the requirements of the tube, the diameter d, in particular, beingdetermined by the gap modulation coefficient required (i.e. the factorusually designated by 18). In a system in which L is 5 cm. and d 1.8mm., the diameter D would be about 2.5 mm.

As an example of a design of a beam system according to the presentinvention and for comparison with pure electrostatic or magneticfocusing arrangement, let us assume that we wish to limit the beamvoltage to 1000 volts. From Equation 4, in the electrostatic case wefind that the maximum current that can be forced through a tunnel of 5centimeters length is 9.2 ma. In the magnetic case, allowing for a 10%fall in potential in the beam, we find, from Equation 3, that themaximum current is 197 ma. Thus the eleectromagnetic focusing gives avery much higher current but the cathode loading for such a currentwould be prohibitive, since, assuming a cathode diameter of 2.5 mm.(equal to the main tunnel) it would be at least 4 a./cm.'. Actually thecathode would have to be a little smaller than this, with acorresponding increase in current.

With the present invention, using the arrangement of FIG. 2, we mayretain our 197 ma. but reduce the cathode loading. To the left of thegap 12 we have electrostatic focusing and to right magnetic. Forcontinuity of current we equate I and I in Equations 3 and 4 and soobtain the required length of electrostatic tunnel to produce a parallelbeam at the gap 12. One half of the electrostatic tunnel is required.For the dimensions previously given, with 1000 volts beam potential, wefind that the pole piece width U2 is approximately 5.4 mm. It remains todesign a suitable electrostatic electron gun, according to knownprinciples with which the present invention is not concerned, which willproject about 200 ma. current at'1000 volts into the tunnel aperture. Byway of exemplifying the decrease in cathode loading it is known that foran optimised arrangement of a gun designed according to the principlesdisclosed by I. R. Pierce, commonly known as a Pierce gun, the currentdensity at the electrostatic throat is approximately 25.5 times that atthe cathode. Using the figures in the above example the current densityat the throat is about 8 a./cm. so that the cathode loading would onlybe about 0.32 a./cm. which is well within the handling capacities ofmodern oxide-coated cathodes. If we assume 1 21./cm. as a reasonablemaximum cathode loading it is clear that current densities of 25.5a./cm. can be obtained in the working part of the beam. The Pierce gunis used as an example because it is readily amenable to theoreticaltreatment. Other guns can be used equally or more effectively and thereis no reason to assume that 25.5 is the maximum ratio of cathode area tothroat area which can be obtained.

In the embodiment of FIG. 3, the invention is shown applied to a tworesonator klystron. Thetunnel 9 of FIG. 2. is replaced by a drift tube19, separating two resonant cavities 20 and 21. These cavities areformed between pairs of metal discs 22 and 23, the disc 22 forming theouter wall being made of nickel-iron alloy having substantially the samecoefficient of expansion as glass, such as that sold under theregistered trademark Cinseal," while the disc 23 may be of copper. Thediscs are clamped to annular metal collars 24 and 25 which may beprovided with tuning screws 26 for adjusting the resonant frequency ofthe cavities 20 and 21. Glass sleeves 27, 28 and 29 are sealed betweenthe respective discs as shown to form an envelope for the device.Tubular extension members 30 and 31 secured to the respective plates 22are aligned with the drift tube 19 and form continuations thereofleaving gaps 32 and 33 in which interaction between the electron beamand the electromagnetic field .in the respective cavities, 20 and 21 mayoccur. Hollow pole'pieces 34 and 35, corresponding to the pole pieces 10and 11 of FIG. 2, are secured to the respective plates 22 at either endof the device. The pole piece 34 houses an indirectly heated cathode 36and a focusing cylinder 37, which components are shown diagrammaticallyon the drawing. The hermetic enclosure of the pole piece 34 is completedby an alloy skirt 38 sealed to the material of the pole piece and to aglass base 39 carrying an exhaust tubulation 40 and leads 41 for theelectron gun electrodes. Pole piece 35 is closed by an end plate 42which forms the collector electrode for the electrons projected throughthedevice. The magnetic circuit is completed by means of a permanentmagnet or solenoid indicated at 43 and coupled by means of yoke arms 44to the respective pole pieces. The dimensioning and operation of thedevice will be evident to those skilled in the art from the foregoingdiscussion with reference to FIG. 2. It is important to note however,that the pole piece 34 must be of suflicient thickness to provideadequate magnetic screening for the electron gun.

In the embodying of FIG. 4, the invention is shown as applied to atravelling wave tube. The tube comprises a glass sleeve 46 which issealed by short Cinseal cylinders 4,7 to the pole pieces 48 and 49. Ahelix 50, terminating in pick-up posts or probes 51 and 52 secured tothe pole pieces, is positioned in the sleeve 46 by means of quartzsuport rods 53 which are located in the pole piece and faces. Thetravelling wave tube projects at either end through rectangular waveguides indicated at 54 and 55.

The pole piece 48 houses an electrostatic gun comprising an indirectlyheated cathode and focussing cylinder shown diagrammatically at 56 and57 respectively, these members being located by mica washers 58. As inthe previous example, the pole piece 48 is closed by means of an alloyspinning 59 sealed to a glass base 60 carrying an exhaust tubulation 61and electrode leads 62. At the other end of the tube the pole piece 49comprises a cylindrical member 63 in which iron or Cinseal blocks 64 and65 may be brazed, these blocks being hollowed out to provide a chamber66 the walls of which constitute the electron collector electrode.

As in FIG. 2 the magnetic circuit is completed by yoke arms 67 and apermanent magnet or solenoid 68.

While the principles of the invention have been described above inconnection with specific embodiments, and particular modificationsthereof, it is to be clearly understood that this description is madeonly by way of example and not as a limitation on the scope of theinvention. In particular the same principles may be used to obtainrectangular cross-section beam of high current density, if cylindricallenses are substituted for the spherical lenses discussed above.

What I claim is:

1. An electron discharge device having an electron beam source, a tunnelmember for the flow of an electron beam therethrough, and means toprovide a magnetic field coaxially of said tunnel member to minimizedivergence of said beam within said tunnel member, said beam sourcecomprising an electron emitter of materially greater cross-sectionalarea than that of said tunnel member, electrostatic focussing means toconverge the electrons from said emitter into a beam of across-sectional area to enter into said tunnel member for substantiallyparallel flow at the entrance thereof, and a shield of magnetic materialfor shielding said beam from influence by said magnetic field prior toentrance into said tunnel member.

2. An electron discharge device according to claim 1, wherein the meansfor producing said magnetic field includes a pair of pole pieces atopposite ends of said tunnel and said electrostatic focusing means iscontained within one of the pole pieces.

3. An electron discharge device according to claim 2, wherein said onepole piece is hollow and has an aperture forming a second tunnel memberadjacent the entrace and coaxially of the aforementioned tunnel member,said second tunnel member constituting said electrostatic focusingelectrode for said beam and the end of said one pole piece adjacent saidsecond tunnel member forming said magnetic shielding means.

4. An electron discharge device according to claim 3, further includinga pair of cavity resonators, one adjacent each pole piece and incommunication with the ends of said first tunnel member.

5. An electron discharge device according to claim 3, further includinga pair of wave guides one adjacent each pole piece and in communicationwith the ends of said first tunnel member, and wherein the meansdefining said first tunnel member includes a helical wave conductor.

6. An electron discharge device having an electron beam source, a tunnelmember for the flow of an electron beam therethrough, said beam sourcecomprising an electron emitter, said tunnel member at no point having agreater cross-section than that of said electron emitter, means to focussaid electron beam into a beam of given diameter, means to provide amagnetic field at a given distance from said source to maintain saidgiven beam diameter constant, and means to shield said source frominfluence of said magnetic field surrounding at least a portion of saidbeam in the region of said source.

7. An electron discharge device having an electron beam source, a tunnelmember for the flow of an electron beam therethrough, said beam sourcecomprising an electron emitter, said tunnel having a uniformcross-section less than that of said electron emitter, means to focus anelectron beam into a beam of given diameter prior to entrance into saidtunnel member, means to provide a magnetic field at a given distancefrom said source to minimize divergence of said electron beam, and meansto shield said source from influende of said magnetic field surroundingat least a portion of said beam of said source.

References Cited in the file of this patent or the original patentUNITED STATES PATENTS 2,225,447 Haefi et al. Dec. 17, 1940 2,300,052Lindenblad Oct. 27, 1942 2,305,884 Litton Dec. 22, 1942 2,376,707 McCoyMay 2, 1945 2,516,944 Barnett Aug. 1, 1950 2,524,252 Brown Oct, 3, 19502,567,674 Linder Sept. 11, 1951 2,579,654 Derby Dec. 25, 1951 2,591,350Gorn Apr. 1, 1952 2,608,668 Hines Aug. 26, 1952

